The present disclosure is directed to methods for reducing the levels of cyclic oligomers produced during the formation of polyetherimide resins. More particularly, sterically hindered substituted aromatic diamines are utilized which result in lower formation of undesirable cyclic oligomer by-products. Polyetherimides produced in accordance with the methods disclosed herein are also provided.
Aromatic polyethers, particularly polyetherimides, are important engineering resins because of their excellent properties. One route for the synthesis of these polymers is by the reaction of salts of dihydroxyaromatic compounds, such as bisphenol A disodium salt (BPA.Na2), with dihaloaromatic molecules. For example, polyetherimides are conveniently prepared by the reaction of salts of dihydroxyaromatic compounds with bis(halophthalimides) as illustrated by 1,3-bis[N-(4-chlorophthalimido)]benzene (hereinafter sometimes “ClPAMI”), which has the structure: 
The bis(halophthalimides), in turn, are frequently produced by reacting at least one diamino compound, preferably an aromatic diamine such as m-phenylenediamine (mPD) or p-phenylenediamine (pPD), and at least one halophthalic anhydride such as 3-chlorophthalic anhydride (3-ClPA), 4-chlorophthalic anhydride (4-ClPA), dichlorophthalic anhydride, or mixtures thereof.
According to U.S. Pat. Nos. 5,229,482 and 5,830,974, the preparation of aromatic polyethers may be conducted in solution in relatively non-polar solvents, using a phase transfer catalyst which is substantially stable under the temperature conditions employed. Solvents disclosed in U.S. Pat. No. 5,229,482 include o-dichlorobenzene, dichlorotoluene, 1,2,4-trichlorobenzene and diphenyl sulfone. In U.S. Pat. No. 5,830,974, monoalkoxybenzenes such as anisole, diphenylether, or phenetole are employed. Solvents of the same types may be used for the preparation of the bis(halophthalimide) intermediates.
The polyetherimides produced by these displacement polymerizations have a relatively high polydispersivity, ranging from about 2.6 to about 3.6, depending upon the amount of 3-ClPA and 4-ClPA used in preparing the ClPAMI monomer. The general scheme for the production of bis(halophthalimide) and the subsequent production of polyetherimide is set forth in FIG. 1.
When bisphenol A, mPD and 4-ClPA are used to produce polyetherimides, it has been found that the level of cyclic oligomers in the final product is about 3%. However, it has been found that the amount of cyclics increases as the level of 3-ClPA is increased as a starting material in ClPAMI synthesis. Where 100% 3-ClPA and mPD are used as the starting material, the amount of cyclic oligomers can range from about 15% to about 20%. Interestingly, it has been found that about two thirds of the cyclic oligomers are a single 1:1 adduct. The reaction scheme demonstrating the use of 3-ClPAMI to produce a polyetherimide with the cyclic oligomer by-product is set forth in FIG. 2.
High levels of these cyclic oligomers can have adverse effects on the properties of the resulting polymer. Such negative effects include a lower glass transition temperature (Tg), reduced ductility, and problems with processing including surface appearance, as demonstrated by reduced glossiness.
In addition, the cyclic by-products, being off specification, must be discarded after separation, increasing the cost and size of the waste stream and reducing the efficiency of the process.
However, it has also been found that the use of 3-ClPA in combination with other bisphenols and diamines can produce polyetherimides possessing higher Tg (about 15° to about 20° C. higher), and improved flow at high shear. It is therefore desirable to use 3-ClPA as a starting material, at least in part, in the production of polyetherimides, but means for reducing the levels of cyclic oligomers are desirable.